Documentation is now handled by the same processes we use for code: Add something to the Documentation/ directory in the coreboot repo, and it will be rendered to https://doc.coreboot.org/. Contributions welcome!
SELF stands for Simple Executable Loader Format. It is a very simple take on the standard [ELF] executable format. It is proposed that SELF be the the standard format for payload code stored in a LAR file and loaded and executed by coreboot-v3. The bayou chooser will also load and run payloads in the SELF format.
Each SELF file is defined as a group of different segments. Each segment either loads data into memory, zeroes a section of memory, or provides information to coreboot or the payload. The segments can be one of the following types:
| ID | Segment | Description |
|---|---|---|
| 'CODE' (0x45444F43) | CODE | This segment has executable code to be copied from the data section to memory. Each CODE segment has a corresponding block of data, which may be compressed in the LAR. Multiple segments of this type are allowed. |
| 'DATA' (0x41543144) | DATA | This segment has non-executable data to be copied from the data section into memory. Each DATA segment has a corresponding block of data, which may be compressed in the LAR. Multiple segments of this type are allowed. |
| 'BSS ' (0x20535342) | BSS | This segment defines a section of memory to be zeroed. BSS segments do not have any data associated with them. Multiple segments of this type are allowed. |
| 'PARA' (0x41524150) | PARAMS | This segment contains the contents of the .notes.pinfo section from the payload ELF. The payload will store parameters in this section for the benefit of a chooser payload. The parameters will be stored as a series of strings of the format "key=value\0". |
| 'ENTR' (0x52544E45) | ENTRY | This segment defines the entry point for execution. This type signifies the end of the list of segments, and it must be the last segment defined in the SELF. It is mandatory and can only be used once. |
A SELF file is comprised of two parts: the segment table and the data section. Each is described below.
The segment table is located at the start of the SELF file, with one entry per segement. Each segment is identified by its type (as listed above). The structure of the header entry is as follows:
struct self_header {
unsigned long type;
unsigned long offset;
unsigned long long load_addr;
unsigned long len;
unsigned long mem_len;
};
All fields are present in each segment entry, though not every field will be valid for every segment type. The value of each field will be stored in little endian format. Big endian architectures will need to byte swap the value before using it. The following table lists which members are used and what they mean:
| Type | offset | load_addr | len | mem_len |
|---|---|---|---|---|
| CODE | Offset of the segment in the SELF file | address where the code should be copied in memory | length of the data in the segment data section | length of the data in memory |
| DATA | Offset of the segment in the SELF file | address where the code should be copied in memory | length of the data in the segment data section | length of the data in memory |
| BSS | N/A | Start address of the block in memory to be zeroed | N/A | Length of the block in memory to be zeroed |
| PARAM | Offset of the segment in the SELF file | N/A | length of the data in the segment data section | N/A |
| ENTRY | N/A | Address where execution should start | N/A | N/A |
The data section immediately follows the final entry in the segment table. Each block of segment data is written sequentially in this section, with the start of each block aligned to a 32 bit boundary. CODE and DATA sections may be compressed according to the LAR compression scheme, the PARAM section must be uncompressed.
The following pseudo code shows how a SELF is loaded and run:
decompress_and_run(void) {
ptr = SELF_start;
while(1) {
header = (struct self_header *) ptr;
switch(header->type) {
case TYPE_CODE:
case TYPE_DATA:
dlen = decompress(SELF_start + header->offset, header->load_addr, header->len);
memset(header->load_addr + dlen, 0, header->mem_len - dlen);
break;
case BSS:
memset(header->load_addr, 0, header->len);
break;
case LOAD:
return jump_to(header->load_addr);
}
ptr += sizeof(*header);
}
}